Non-volatile memory devices such as flash memory devices are used to store data or information. Many computers and electronic devices, for example, digital audio players, digital cameras, digital recorders, and cellular phones, have flash memory devices. Flash memory devices may also be used as portable storage devices such as portable Universal Serial Bus (USB) flash drives or “thumb” drives. Flash memory devices may not need power to maintain the information stored in the device.
A flash memory device stores information in numerous memory cells, which are usually formed in a semiconductor chip. Each of the memory cells often has a metal-oxide semiconductor (MOS) transistor with two different transistor gates: a control gate and a so-called “floating” gate. The control gate may be used to turn the transistor on and off to control access to the memory cell. The floating gate may be the place where one or more bits of information is stored in each memory cell.
The value of the information stored in the floating gate may be determined by the amount of electrons or charge in the floating gate. For example, the information stored in the floating gate may have a first value (e.g., logic zero or binary 0 value) when an excess amount of electrons is present in the floating gate and a second value (e.g., logic one or binary 1 value) when the excess amount of electrons is absent from the floating gate. The presence or absence of the excess amount of electrons in the floating gate may be controlled by varying the amount of electrons in the floating gate, for example, by either adding electrons to or extracting electrons from the floating gate.
A flash memory device usually has a programming or write operation to add electrons to the floating gate, and an erase operation to extract electrons from the floating gate. Information in the memory cells may be read in a read operation. Programming, read, and erase operations in a conventional flash memory device usually involve applying voltages to the memory cells, such as to the control gates of the transistors and to other device components within the flash memory device.
A conventional flash memory device may go through many programming, read, and erase operations during its life. Thus, improper control of the voltages applied to the memory cells during these operations may lead to inferior device performance, reliability, or both.